Ultra-violet gas analysis apparatus to determine the relative gaseous concentration in an anesthetic system



1965 A. D. ROB1NSON 3,166,676

ULTRA-VIOLET GAS ANALYSIS APPARATUS TO DETERMINE THE RELATIVE GASEOUS CONCENTRATION IN AN ANESTHETIC SYSTEM 2 Sheets-Sheet 1 Filed Feb. 24, 1961 INVENTOR. 4L F850 A .05/N50/V A. D. ROBINSON Jan. 19, 1965 3,166,676 ULTRA-VIOLET GAS ANALYSIS APPARATUS TO DETERMINE THE RELATIVE GASEOUS CONCENTRATION IN AN ANESTHETIC SYSTEM 2 Sheets-Sheet 2 Filed Feb. 24, 1961 4060A; WQl E LEA/6774 mga es4a N w T N my W we r United States Patent Ofiice sunsets ULTRA-VIGLET GAS ANALYSES APPARATUS T6) DETERMKNE T \ELA'ilt E GASEQUS Jill CENTRATIUN EN AN ANETLt-LETi SYSTEh-il Alfred ll). Robinson, El Monte, Calii, assiguor to Analytic Systems Company, Pasadena, ilaiiii, a corporation of California Fiied Feb. 24, 1961, er. No. 91,428 6 (Ilairns. (Cl. flit- 235) This invention relates to apparatus for use in inhalation anesthesia and more particularly to a system for measuring the amount of halothane or other dangerous gases employed as a gaseous anesthetic component of an oxygen mixture being supplied to a patient undergoing surgery. a

In inhalation anesthesia, gas containing both oxygen and a gaseous anesthetic agent is supplied to a patient through a supply line to suitable breathing apparatus. In so-called open systems, the gas exhaled by the patient is discarded. In so-called closed systems, the exhaled gas is re-circulated to the supply line after purification. The gas that is returned to the supply line for this purpose is first passed through a purifying device which removes carbon dioxide (CO Without removing the anesthetic gas. In semi-closed systems, a portion of the recirculated gas is bled off to the atmosphere or a waste line. Whenever possible, closed and semiclosed systems are employed to conserve rare and expensive anesthetic gases.

Some anesthetic gases, while having some advantages, are dangerous to use because of the fact that they may seriously or even fatally poison or otherwise injure a patient if used in excess concentrations. When the concentrations that may be employed lie within critical limits, it is dangerous to employ such gases in closed or semiclosed systems unless a reliable measuring system is available for accurately indicating the concentration of the anesthetic gas.

Halothane (2-bromo-2-chloro-l 1 l-trifiuoroethane) is one of the most desirable anesthetic gases to use in inhalation anesthesia. This gas is most commonly known by the trademark FLUOTHANE registered by Aye-rst Laboratories, Inc. Haloth-ane is very advantageous to employ, compared with many other anesthetic gases because of its non-explosive character, its high degree of potency and the ease with which anesthesia may be induced and the ease of recovery. However, like other halogenated anesthetic gases, halothane is a cardiac depressant, thus requiring special care in its use.

In utilizing halothane, anesthesia is effectively induced by supplying a patient with oxygen containing 2% halothane for a few minutes. After anesthesia has been in duced, the concentration of halothane is reduced to about /2% to 1% in order to maintain anesthesia for a sustained period sufficiently long to enable an operation to be performed. Throughout its use, the concentration of halothane is maintained below about 2.5 to 3.5%, since exposure of a patient to halothane of that concentration or higher for any sustained period may prove to be lethal. In the absence of an accurate reliable system for measuring halothane concentration in a closed system, the concentration of halothane may easily increase beyond a safe limit as the halothane is recirculated over and over from the exhaust line to the supply line. The narrow range of concentration of halothane that is suitable to use has made it practically impossible to employ halothane in a closed system heretofore. This is a serious disadvantage since the current cost of halothane is about $200.00 per liter and since as much as 50 cc. of halothane may be 3,156,676 Patented Jan. 19, l9b5 centration of halothane between a lower limit of about /z% to about 1% and an upper limit of about 3% throughout an operation.

Several attempts have been made to measure the concentration of haloth'ane present in the recirculating line of a closed circuit system. One method has taken ad vantage of the infrared absorption characteristics of halothane and more particularly the fact that halothane has an absorption band at 1.69 Another method has involved the fact that the velocity of transmission of sound of a gas containing halothane, is reduced by the presence of halothane because of the fact that halothane has a high density compared with other gases. Both of these methods have proved to be unsatisfactory. On' the one hand the sound velocity method has proved to be unreliable because the amount of change in velocity produced by a smail change in the concentration of halothane is small and is often masked by changes caused by the pressure of water vapor and carbon dioxide. in addition the measurement of the attenuation of the infra-red radiation at the 169 band requires the use of very expensive and complicated equipment.

I have discovered a very simple system which is useful for measuring the concentration of halothane in pure 0xygen or in mixtures of oxygen with other carrier gases. This discovery involves the fact that an ideal relationship exists between the absorption spectrum of halothane, the emission spectrum of a mercury vapor lamp and the absorption spectra of other gaseous components likely to be present in gas used in anesthesia. More particularly, I have found that halothane has substantial absorption at the wavelength of the 2537 A. resonance line of mercury and that other gases that might otherwise lead to spurious results are free of absorption at that wavelength. Thus, while halothane has substantial absorption at 2537 A. units, oxygen (0 carbon dioxide (CO and nitrous oxide (N 0), the gases that are most likely to be present therewith, are free of absorption at that wavelength.

Based upon the discovery of the relationship between the absorption spectrum of halothane, other gases likely to be present in a system employed in inhalation anesthesia and the emission spectrum of a mercury vapor discharge lamp, i have devised a very simple, relatively inexpensive, highly reliable system for continuously indicating the concentration of halothane in gas being supplied to a patient. This system is so reliable that it makes halothane safe to use in a closed or semi-closed system, as well as in an open system.

in the best mode of practicing the invention now known, the absorption characteristic of the gas mixture is measured in a narrow band in the ultra-violet region of the absorption spectrum of the anesthetic gas. The band used is a very narrow band, such as that represented by a spectral line. This narrow band, or line, lies on the absorption side of the edge of the ultraviolet absorption spectrum of the anesthetic By employing a very narrow band of radiation, a high degree of linearity can be produced between the reduction in intensity ofthe radiation passed through the gas mixture and the concentration of the anesthetic gas. My invention takes advantage of the fact that the intense 2537 A. of the line spectrum in the ultraviolet spectrum of a mercury lamp lies within the absorption spectrum of halothane near the absorption edge. in the best mode of practicing the invention now known, the sample undergoing test is continuously drawn from a recirculating line of a closed system so as to facilitate the provision of a continuous indication of the concentration of halothane in the gas being supplied to the patient.

in a specific system incorporating my invention the use of a mercury vapor lamp and means including a filter for selectively transmitting radiation having a wavelength of 2537 A. from the lamp through a sample of gas withdrawn from the supply, exhaust, or recirculating line, together with means for measuring the degree to which such radiation is absorbed by the specimen.

I have also found that the invention is useful when a mercury vapor lamp is employed to detect another anesthetic gas, trichloroethylene, sometimes known by the trademark Trilene. This gas, like halothane, has an absorption edge near the intense mercury 2537 A. line. This gas, like halothane, has a moderate absorption coefficient at this wavelength, thus making it, like halothane, ideally suited for use in measurement with path lengths in the centimeter range. Trichloroethylene, however, unlike halothane, cannot-be readily used in a closed system. The reason for this is that trichloroethylene reacts with soda-lime absorbers that are generally employed in closed systems to absorb the carbon dioxide (CO that is exhaled by the patient. More particularly, trichloroethylene is unsatisfactory to use in a closed system because some of the reaction products formed by it in passing through a soda-lime absorber can produce severe brain damage. sometimes satisfactory to use in an open system.

In addition, some of the features of this invention find use with other anesthetic gases, such as nitrous oxide, ethyl ether, ethyl chloride, and chloroform. The latter gases, however, do not have any substantial absorption at a wavelength of 2537 A. For this reason, with these gases, radiation of other wavelengths is used. Such radiation may be supplied by'a hydrogen arc source through a monochromator.

Since halothane is employed in the best mode of practicing the invention now known, the invention will be described hereinafter primarily with specific reference to the use of halothane. However, it is to be understood that many features of the invention may be employed with other anesthetic gases.

The foregoing and other aspects of this invention will be explained more fully in connection with the description of a specific embodiment of my invention. It is therefore to be understood that other features than those mentioned above are utilized in various forms of the invention and that the invention has other advantages than those specifically mentioned above. The invention will be understood by reference to the following description taken in connection with the accompanying drawings in which:

FIGURE 1 is a schematic drawing of an embodiment of the invention;

FIGURE 2 is 'a detailed schematic diagram of a cell and the associated circuitry employed for determining the concentration of halothane present in the gas under test.

FIGURE 3 is a graph showing the relationship between the spectral characteristics of halothane and various parts of the system.

General description of inhalation anesthesia system Referring to the drawing and more particularly to FIG. 1, there is shown a complete inhalation anesthesia system embodying my invention. Though the invention itself resides in the combination of the halothane detector with the remaining portion of the inhalation anesthesia system, it is deemed to be desirable to first explain the system as a Whole in which the halothane detector is employed.

In the inhalation anesthesia system illustrated in FIG. 1, oxygen and other gases are supplied in regulated amounts to a mask applied to the face of a patient. The gas supply system illustrated includes an oxygen supply tank and a nitrous oxide supply tank 46 connected respectively through pressure regulators 31 and 41 to a mixing valve 5% which feeds gas through a second Nevertheless, trichloroethylene is mixing valve 52 to a supply line 69 connected to the inlet 61 of a /-valve 62 mounted on the breathing mask 2%. The flow of oxygen from the oxygen tank 349 through the main oxygen flow line to the mixing valve 50 is controlled by valve 35 and flow meter 34. In a similar way, nitrous oxide flows from the tank 40 through the pressure regulator 41, the valve 4-3, the flow meter .4 in a nitrous oxide flow line to the mixing valve 50. The mixture of oxygen and nitrous oxide then flows through a line 51 to the mixing valve $2 to the supply line 69. Under many conditions the nitrous oxide is shut oif completely by closing the valve 43. In any event with the gas supply system just described, oxygen can be flowed at a regulated rate through the valve 52 into the supply line 6%, either with or without an analgesic gas such as nitrous oxide being added.

A branch line 55 is employed for supplying a small proportion of a volatile anesthetic gas, more particularly halothane, to the supply line 6%). The line '55 includes a valve 53, a flow meter 54 and a vaporizer 56 con nected in sequence between an inlet fitting 36 and the mixing valve 52. The inlet fitting 36 is connected between the pressure regulator 31 and the valve 33.

The vaporizer 56 is normally in the form of a cylindrical vessel in which a charge 57 of liquid halothane is placed and in which vapor is sealed by means of a cap 59 screwed or otherwise clamped in place. The inlet section of line 55 is connected to an inlet fitting 72 on one side of the vaporizer and the outlet section of line 55 is connected to an outlet fitting '73 on the other side of the vaporizer. With thi arrangement, so long as oxygen flows in the line 55, vapor in the space above the charge 57 of liquid halothanc is continuously swept towardthe valve 52 where it is blended with the gas flowing thereto through the line 51. The mixture formed in the valve 52 contains oxygen from the oxygen tank 30, halothane from the vaporizer 56 and any other gas that has been introduced from an auxiliary source such as 40. This mixture flows through the supply line 60 to the mask 20.

The Y-valve includes a check-valve system such as a check-valve 61 connected between the supply line and Y-valve for permitting gas to flow from the supply line into the mask 2t and a check-valve 63 to permit gas to flow out of the mask 21 into an exhaust line 65. When in use, gas that is to be inhaled by the patient is supplied to the mask through the supply line and gas that isexhaled by the patient flows out of the mask into the exhaust line 65. The mask itself may be of any suitable type -adapted to fit the face of the patient undergoing anesthesia.

It is useful to use nitrous oxide with fluotha-ne since this reduces the amount of halothane needed to attain satisfactory anesthesia. Helium from another tank (not shown) is sometime added as a diluent.

Since halothane has a substantial vapor pressure at room temperature and since in any event it is important to maintain the temperature of the breathing equipment at a temperature close to room temperature and since room temperatures vary over a wide range in places, such as hospitals, where patients are likely to be treated, the rate at which halothane is supplied to the supply line 60 likewise varies over a Wide unsafe range, unless adequate precautions are taken to limit the concentnation of halothane to a narrow safe range. As mentioned above, the concentration of halothane should be maintained between about /2 of 1% to about 2 or 2 /2%. Both the lower limit and the upper limit can'be adjusted somewhat under the control of an experienced anesthetist in order to induce anesthesia safely in the patient'heing subjected to .the anesthesia provided that he ha available some. method of measuring the concentration.

In practice oxygen flows through line at a much thane, the concentration of halothane fed to the supply line 60, is low.

In an open system, the exhaust line 65 communicates with a waste line 67 that leads the exhaust gas to the atmosphere outside of the hospital. When the system is utilized as a closed system, the exhaust line 65 conneots through a decarbonator, that is a carbon dioxide absorber 66, to a portion of the supply line 60 between the mixing valve 52 and the check-valve 61. In this way the exhaust line 65 becomes a recirculating line which causes gas exhaled by the patient to flow through the decarbonator 66 to the supply line 60. A cannister containing soda-lime is suitable for use as a decarbonator. The decarbonator selectively removes carbon d-ioxide without removing oxygen or halothane. A pop-oft valve 64 located in the exhaust line between the checkvalve 63 and the decarbonator prevents the pressure in the system from exceeding atmospheric pressure by any substantial amount. A manually operated selector valve SV located between the pop-off valve the the decarbonator may be used to convert the system to an open system. The selector valve may be a twoway valve for selectively connecting the exhaust line 65 to the decarbonator 66 when in its normal position or to the waste line when in its auxiliary position.

At the point at which the exhaust line 65 returns to the supply line 60, it is connected to a fitting 76 which has a closed breathing bag 72 depending therefrom. The fitting permits free communication between the exhaust line 65 and the supply line 66 between the valve 52 and the valve 63 and also free communication between these lines and the breathing bag.

With the selector valve in its normal condition, when the selector valve SV is open, the system operates as a semi-closed system. When it is closed, the system operates as a closed system. When the return connection between the exhaust line 65 and the supply line is removed or the safety valve is turned to its auxiliary position, the system is called an open system. Except where otherwise specified hereinafter, the system will be described as a closed system.

When the system is in use, the valves 33, 43 and 53 are open. For example, it is common to set the valve 33 at a point to cause oxygen to flow through the system at the rate of about 8 to 10 liters per minute when the system is operated as an open system and at a rate of about 400 cc. per minute when operated as a closed system, and at an intermediate rate when operated as a semiclosed system. The flow meter 34 is employed to indicate the rate of flow of oxygen in the main oxygen flow line 35. The valve 53 is adjusted to a position suitable for causing oxygen to flow through the branch line 55 at a rate that is small compared with the rate of flow of oxygen through the main line 51. The rate of flow in the branch line is indicated by flow meter 54. The valve 43 is adjusted to a position suitable for causing nitrous oxide to flow through the flow system at suitable low rate as indicated by fiow meter 44. Nitrous oxide is almost never used when the system is operated as a closed system. But when the system is operated as an open system the rate of flow of nitrous oxide is usually maintained between 2.5 and 4 times the rate of flow of oxygen. In this way a high proportion of nitrous oxide is made available to the patient without reducing the partial pressure of oxygen below the value that it has in the atmosphere.

The oxygen and any nitrous oxide present that enter the mixing valve 50 flow through the line 51 to the second mixing valve 52 when they meet with the halothane mixture flowing from the branch line 55. The mixture of oxygen, nitrous oxide (if any) and halothane blend in the mixing valve 52 where they enter the supply line 66. Gas flowing through the supply line 60 enters the Y-valve 62 through the check-valve 61, thus supplying the mixture of gas to be inhaled by the patient to the mask. Gas exhaled by the patient is exhausted through the checkvalve 63 into the exhaust line 65, with the safety valve in its normal position (in which case no .nitrous oxide is supplied). The gas flowing in the line 65 enters the decarbonator 66 where carbon dioxide is removed. The purified gas includes residual oxygen that was not consumed by the patient and also the halothane that was not absorbed in the patients system. The halothane and the residual oxygen entering the fitting 70 blend with the gas flowing through the mixing valve 52, and thereby reenter the supply line 60.

The breathing bag 72 expands and contracts as the patient exhales and inhales gas respectively, thereby permitting gas to flow through the mixing valve 52 at a nearly constant rate even though gas is flowing intermittently through the supply line into the mask 20. In cases where a patient needs assistance in breathing, the anesthetist squeezes the bag to force gas into the mask.

In a closed system, the concentration of halothane gradually increases in the gas being supplied to the patient through the supply line 66. As previously mentioned, an excess of halothane above about 2 /2 to 3 /z% can be lethal. To prevent an excess concentration of halothane in the gas supplied to the patient and to maintain the halothane concentration at any lower levels required, a halothane concentration measuring system 10 is used.

Halothane measurement In the best mode of practicing the invention now known, a small fraction of the gas flowing in the supply line 60 is continuously flowed through the halothane detector 10. In the simplest way of achieving this result, a test cell 1% in the halothane detector is connected to a line 86' which leads on the upstream side of the detector to the supply line 60 and to a line 82 which leads on the downstream side through a bleeder valve BV and a trap 84 to a suction pump SP which draws gas from the cell through the bleeder valve and discharges it to a waste line or vent 86. By mounting the bleeder valve BV on the downstream side of the cell, the gas in the cell is maintained at a pressure nearly equal to atmospheric pressure. In this way, the partial pressure of halothane in the cell is rendered substantially equal to the partial pressure of halothane in the supply line.

Normally, the bleeder valve is adjusted to such a position that about 100 cc./min. of gas is drawn through the cell each second. In this way, the halothane detector responds rapidly to changes in halothane concentration comparable to the time elapsed between successive inhalations of gas by the patient. Rapid fluctuations in halothane concentration are largely smoothed out by virtue of the action of the decarbonator 66 and the breathing bag 76. With the 15 cc. sample cell employed, the time constant of the system was then about 15-20 sec. By drawing gas out of the system at a flow rate of about 100 cc./min., the amount of halothane wasted is maintained very low.

The cell 100 is in the form of an elongated cylindrical tubular member having transparent end Walls 162 that are flat and parallel. The flow line 86 is connected to tubular members and 82 projecting laterally from the cylindrical side Wall 103 as shown in FIGS. 1 and 2. While in practice, the end Walls 102 need not be transparent to visible light, it is important that they be transparent to the radiation which is being used for measuring the absorbance of the gas. End walls composed of fused quartz are suitable.

As illustrated in FIGS. 1 and 2, a mercury vapor lamp 106 and a photodetector 108 are mounted externally of the cell opposite the two end walls 162 so that radiation emitted from the mercury vapor lamp 106 is transmitted to the photodetector 168. The radiation is transmitted as a beam having a central axis 116 that passes through the cell 166 along a path extending through the end walls 102. A filter is arranged between the discharge lamp 106 and the photodetector M38 in order to triodes 1117 and 117 are at the same potential.

selectively transmit radiation having a Wavelength of 25 37 i A. from the discharge lamp through the sample to the photodetector 108. The filter 1111 may be used as an end Wall. A circuit 112 is operated by the photodetector 1 .18 to produce an indication on a meter 1114 representing the percentage concentration of halothane in the supply line 619. a

In the best embodiment of the invention as indicated in more detail in FIG. 2, a second photodetector 113 and a filter 120 are employed in order that compensation may be made for any changes that occur in the intensity of the 25 37 A. line of the mercury vapor lamp from time to time or during operation. In this case the meter 114 is connected in a special comparison circuit 112 shown in FIG. 2. In the comparison circuit, the two photodetectors 108 and 118 are connected to the input circuits of a balanced amplifier that employs two cathode-loaded triodes 107 and 117 for producing a differential voltage proportional to the absorption of radiation in the cell. The anode A of each of the photodetectors is connected to the 3+ terminal or" a power supply (not shown) while the cathode K of each is connected through resistors R and R to the grounded negative terminal of the power supply. One of these resistors is made adjustable to facilitate balancing the amplifier circuit. The two triodes 197 and 117 are provided with cathode resistors R and R The anodes a of the triodes 107 and 117 are also connected to the B+ terminal, while the cathodes k are connected to the ground terminal through cathodeloading resistors R and R The control grids g of the triodes are connected to the respective cathodes K of the corresponding photodetectors 108 and 118.

The current meter 114 is connected in series with an adjustable resistor R between the two cathodes k of the triodes 107 and 117. This meter 114 and resistor R respond to the difference in the voltages across the two resistors R and R and therefore may be considered as acting as a voltmeter.

With the circuit illustrated in FIG. 2, a voltage difference is developed between the cathodes k of the triodes 1t)? and 117 proportional to the difference in currents flowing through the corresponding photodetectors 1118 and 118. Since each of these currents is proportional, or at least very nearly proportional to the intensity of the radiation striking the respective photodetectors, the potential diiference between the cathodes k is proportional to the amount of radiation absorbed by passage of light through the cell 1110. g

In order to calibrate the meter 114 to provide a reading which accurately indicated the concentration of halothane in the gas in the cell 100, a calibrator in the form of a calibrating mask or'calibrator 131i is arranged to be positioned between the discharge lamp 106 and the photodetector 168. This calibrator which is normally removed from the path, may be inserted therein by manipulation of an arm 131 that projects through the case or cabinet 12 in which the cell 1% and the circuit 112 and other associated parts, are mounted. The calibrator 1311 may be in'the form of a neutral density plate or other partially transparent filter or it may be in the form of an apertured opaque sheet or a screen In any event the optical calibnator 130 is employed in connection with certain adjustable parts of the amplifier 112' to calibrate the instrument.

More particularly, in order to calibrate the instrument, the calibrator 130 is initially removed from the path 116 so that it does not interfere with the transmission of radiation from the discharge lamp 1% to the photodetector 108. With the calibrator 1311 thus removed and with gas free of halothane flowing through the cell 109,

the value of the resistor R is adjusted to a balanced, or null, position in which no current flows through the meter 114, that is to the point at which the cathodes k of the Then the calibrator 130 is moved into a predetermined position on the path 116 so as to reduce the total amount of light transmitted fromthe discharge tube 1% through the photodetector 108 by a predetermined fraction. With gas free of halothane still flowing through the cell, the rheostat R in series with the meter 114, is adjusted to produce a predetermined reading such as 2% correspond ing to the concentration of halothane which would produce the same reduction of intensity as is produced by the calibrator 130. In pnactice, a calibrator 130 has pre-. viously been constructed or selected that has a transmission characteristic that corresponds to a predetermined concentration of halothane. Accordingly, it is unnecessary to measure the concentration of halothane in calibrating the instrument, once a suitable calibrator 1311 has been properly calibrated.

For example when employing an apertured calibrator plate 130, one way to make the calibrator plate 130 is to first flow air freeof halothane through the cell and to adjust the value of the resistance R to produce a zero indication by the meter 114, as previously described. Then gas containing a predetermined concentration of halothane such as 2% is flowed through the cell and the value of the resistor R is adjusted to cause the meter 114 to produce an indication corresponding to that concentration. Then, without changing the settings of the resistor. R or the rheostat R gas free of halothane is flowed through the cell 1% and a hole is cut in a plate 121 of such size that when the plate is inserted in the path 116, the same reading is produced on the meter 114 as when gas containing the predetermined concentration of halothane mentioned above flowed through the cell.

Expressed differently, once a suitable calibrator 130 such as a calibrator plate has been provided, the instrument can thereafter be recali'brated at any time by first adjusting the resistance R 'to produce a Zero reading on the meter 1 14 when gas free of halothane is flowing through the cell while the calibrator .131) is withdrawn from the path 116. Then, while gas free of halothane' is still flowing through the cell but with the attenuator 130.

located on the light path 1-16, the value of the rheostat R is adjusted to produce a predetermined reading, such as 2% corresponding to the concentration of halothane for which the calibrator has been calibrated. Thereafter, the attenuator 131) is withdrawn from the optical path and the meter i11 is read while gas containing halothane in unknown concentration flows through the cell. Under these circumstances, in view of the previous calibrations the meter reading indicates correctly the halothane concentration of the latter gas.

In a practical embodiment of the invention, the triodes used were those designated by the code symbol 12AX7 and the photodetectors used were those that are designated by the number 935. The voltage provided by the power supply was 130 volts. The resistors R and R had resistance values of about 4.7 megohms while the resistors R and R had values of 30 kilohrns and the rheostat R was adjustable up to a maximum value of 50 kilohms. The filter used was a Corning glass filter No. 9863.

In FIG. 3, graph G shows how the absorption coefiicient of halothane varies as a lfllllOtlOIl of wavelength. The cut off commences at about 3000 A. and the absorption rises rapidly in the neighborhood of the 2537A. unit mercury line. Graph G2 indicates how the absorption coefiicient of the filter varies with wavelength in the same region. Graph G3 shows how the sensitivity of the photodetector varies with wavelength. Though the mercury vapor spectrum has weak lines L L L and L in the sensitive region of the phot-odetector, the filter and photodetector cooperate to selectively detect a strong narrow band of radiation at 2537 A. in a region where the absorp tion coefiicient of halothane is about 0.2 at mm. Hg. As mentioned previously, other gases likely to be present manifest practically no absorption at this position. Graph G1 represents the absorption coefficient for a 1 cm. path length of air saturated with halothane at 20 C. At this temperature, the vapor pressure of halothane is 224 mm. Hg. a

In practice, the intensity of I of radiation striking the photodetector 1108 is related to the intensity 1,, of radiation emitted by the discharge lamp 166 by the following equation:

I =kl e (1) When the contration is low, that is of the order of a few percent or less, the intensity is given by the following approximate equation:

I =kI (1-cdA) This formula is very accurate when cdA is very small compared with 1.

In Equations 1 and 2, the various terms have the following meanings:

A=the absonbance or absorption coefiic-ient of halothane;

d=the length of the optical path along which radiation is transmitted through the gas under test;

c=the concentration of haloth-ane in the gas.

25 X For such concentrations, and lower concentrations, the linear equation given above is sufficiently accurate to indicate the actual amount of radiation absorbed when using a cell having a 10 cm. length.

The coeflicient k of Equations 1 and 2 takes into account the amount of radiation absorbed by the walls 132 of the cell, the filter Elli), etc. In other Words, k is the ratio of the intensity of radiation striking the photodetector ltlli to the intensity of radiation striking the photodetector 118 when oxygen free of halothane is flowing through the cell 1%.

In practice, a heater coil Ltd is mounted within the case 12 at a position spaced from the cell 1% in order to maintain the temperature of the air within the cabinet 12 and hence the temperature of the cell 1% somewhat above atmospheric temperature. In this way condensation of moisture on the end walls 1452 is prevented. Unless such precautions are taken, especially in cold climates the presence of such moisture may absorb and scatter radiation, thus producing spurious indications of halothane concentration. As a matter of fact some rooms where inhalation anesthesia is practiced, whether they be rooms in hospitals or rooms used under emergency conditions, the temperature of the air in the room may lie anywhere in the ran e from 68 F. to 90 F. In some cases, especially in Warmer rooms, the heat gen erated by the heaters 11 that energize the cathodes k or by any power supply enclosed in the cabinet of the instrument is sufiicient to maintain the cell 1% at a sufiiciently high temperature to avoid such condensation of moisture. In either event the use of a cabinet totally enclosing the cell and the heater 146 or the triodes Hi7 and 11,7 aids in making heat available for this purpose.

With this invention, the concentration of halothane flowing in the supply line, may be continuously indicated with a high degree of accuracy. If for any reason the concentration of halothane falls below a maintenance value of about 1%, or exceeds an unsafe value of about 2.5%, suitable steps can be taken to reduce the danger to the patient. For example, the selector valve SV in the re-circulating line may be opened immediately to vent the gas being re-circulated thus converting'the systern from a closed system to an open system so that the concentration of halothane in the supply line 60 may be quickly reduced. Alternatively or simultaneously, the valve 53 in the branch line 55 may be throttled to reduce the rate of flow of halothane from the vaporizer into the supply line 66. In any event, by means of this invention, rapid and accurate determinations of changes in halothane concentration are indicated, thus facilitating reliable maintenance of the halothane concentration within suitable safe limits.

By virtue of the fact that halothane concentrations are accurately and reliably indicated by means of this invention, this invention makes it practical to employ halothane in a closed system. By means of this invention the amount of halothane required to achieve satisfactory anesthesia for a predetermined time interval, is

. greatly reduced, thus greatly reducing the cost of the l thane used. At the present time the price of halothane is $290 per liter. When it is realized that the amount of halothane used per operation when using an system costs about $10 and in some hospitals many thousands of operations are performed each year, it will be appreciated that the use of this invention results in great economy. Furthermore, though this invention finds its greatest utility when used in closed systems, it is also useful in semi-closed and open systems since it is always important to maintain the concentration of halothane within the narrow limits described.

Though the invention has been described with reference to the use of a connection between the halothane detector and the mixing valve 7d that joins the exhaust line 55 with the supply line at it will be understood that the detector can be connected to the inhalation anesthesia system at other points. For example, the halothane detector may be connected through a bleeder valve to a branch of the Y-valve that is located at the face mask. Likewise, it may be connected to any other part of the system.

Other forms of invention As mentioned in the introduction to the specification, this invention finds use not only with halothane, but with other gases, also. More particularly, at least when the analyzer is employed in an open system, it is found that the apparatus of this invention can be employed without any change whatsoever being made in it when trichloroethylene is being used as the anesthetic agent. Graph G!- of FIG. 3 shows how the absorption coefiicient of trichloroethylene varies with wavelength. In this graph, the absorption coeiiicient is given for air saturated with trichloroethylene at 20 C. At this temperature, the partial pressure of halothane is 55 mm. Hg.

It will be noted that trichloroethylene, like halothane, has an absorption edge adjacent the 2537 A. line of the mercury vapor lamps line spectrum. Trichloroethylene, like halothane, has a moderate absorption coefficient at this wavelength that is a coefficient of the order of 6.2 when the partial pressure is about mm. Hg. Such an absorption coeflicient is ideally suited for use in this invention, since such absorption coefiicient makes it possible to measure gas concentrations of a few percent in a cell having a path length in the centimeter range, that, is between about 2 centimeters and about 20 centimeters. Path lengths of this range are especially suitable, since such path lengths are easy to attain within the physical limits of inexpensive compact cell construction.

As mentioned previously, trichloroethylene is too dangerous to use in a closed system which employs a decarbonator using soda-lime for absorbing carbon dioxide. However, it can be used in an open system. The safe range of concentration of trichloroethylene lies between about 0.5% and about 1%. Since the safe range of trichloroethylene concentration is narrower than the safe range of halothane and since trichloroethylene cannot be readily employed in a closed system, halothane is more suitable for use as an anesthetic agent.

In Table I, there are listed a number of other anesthetic agents to which this invention may be applied. All of these anesthetic agents have absorption edges t somewhat below 2500 A. For this reason, none of them has any significant absorption at the wavelength of the mercury vapor gaseous discharge lamp line 2537 A. In the first column of Table I, various anesthetic gases to which this invention may also be readily applied are listed. These gases are nitrous oxide, ethyl ether, ethyl chloride, Vinethene, and chloroform. in the second column, the approximate wavelengths at which the absorption edges'are located have been tabulated. In the third column, there are listed wavelengths M at which the absorption coefficient is about 0.2 for a 1 cm. path length at the vapor pressure at 20 C. for the respective gases.

As mentioned, none of these gases has any substantial absorption coeflicient at a wavelength of 2537 A. For this reason, it is impractical to employ a mercury gaseous discharge lamp as a source with any of them. However, successful measurements may he obtained by employing a source which is in the form of a hydrogen arc with a suitable monochromator for transmitting the radiation of an appropriate wavelength through the sample. Such a monochromator may be a prism monochromator or it may be in the form of a narrow-band-pass filter such as a multi-layer interference filter. For example, when employing chloroform as an anesthetic agent, a filter passing a narrow band of radiation at 2175 A. is employed. Likewise, for example, when employing nitrous oxide as an anesthetic agent, a filter passing a narrow band of radiation at 2100 A. is employed. The use of such a'hydrogen arc and filter, however, is much more expensive than the use of a mercury lamp as described above. For this reason, too, halothane is especially suitable for use in inhalation anesthesia, especially in a closed system.

While the invention has been described with reference to only a few specific embodiments thereof and specific applications thereof, it will now be clear to those skilled in the art that the invention may be embodied in other forms and employed in other ways. It is therefore to be understood that the invention is not limited to the specific forms and applications thereof which have been described herein, but that it may be embodied in other forms and applied in other ways within the scope of the appended claims.

The invention claimed is:

1. In a system of inhalation anesthesia in which a mixture of Oxygen and a halothane vapor are fed through a supply line to a patient and in which a recirculating line is employed for supplying exhaled gas to said. supply line after purification, the combination therewith of:

a gas test cell connected to one of said lines for receiving a sample of gas therefrom; a source of radiation comprising a mercury vapor lamp; a radiation detector;

means associated with said radiation source and said detector for transmitting selected radiation from said 7 source to said detector through a portion of the gas in said cell, said means including an optical filter for selectively restricting the radiation reaching said detector from said source to a narrow band including the 2537 A. of the mercury line spectrum; and

means responsive to the intensity of radiation detected for indicating the concentration of halothane in said mixture.

2. In a system of inhalation anesthesia in which a mixture of oxygen and a halothane vapor are fed through a supply line to a patient, the combination therewith of:

a gas test cell;

means for connecting said cell to said line for receiving a sample of gas therefrom;

a source of radiation;

first and second radiation detectors;

means associated with said source and said first detector for transmitting selected radiation from said source to said first detector through a portion of the gas in said cell, said means including an optical filter for selectively restricting the radiation reaching said first detect-or from said source to a narrow band in the ultraviolet absorption region of halothane; said second detector being positioned to receive radiation directly from said source; and

means responsive to a change in relative intensity of the radiation received by said two detectors for indicating the concentration of halothane in said mixture.

3. In a system of inhalation anesthesia in which a mixture of oxygen and a volatile anesthetic are fed through a supply line to a patient, and in which gas exhaled by the patient is purified without removal of said anesthetic gas and returned to said supply line after purification, the combination therewith of:

a vaporizing vessel connected to said supply line, said vaporizing vessel being adapted to contain a volatile anesthetic in liquid form and to supply said anesthetic in vapor form at room temperature to said supply line;

a gas test cell connected to said line for receiving a sample of gas therefrom;

means for maintaining said cell at a temperature above atmospheric temperature to prevent condensation of moisture in said cell;

means for transmitting radiation through said cell; and

means responsive to a change in the intensity of radiation transmitted through said cell for indicating the concentration of anesthetic vapor in said supply line.

4. In a system of inhalation anesthesia in which a mixture of oxygen and a gaseous anesthetic are fed through a supply line to a patient, said anesthetic having an absorption spectrum with an edge in a region in which other gas components flowing in said line are substantially free of absorption:

means for measuring theabsorption of radiation by said anesthetic gas in a narrow band adjacent said absorption edge, said means comprising a radiant-energy source and a detector mounted on opposite sides of a region through which said sample flows, said source emitting a line spectrum that includes a line within said band and a filter between said source and said detector forselectively restricting the radiation reaching said detector from said source to the line in said narrow band; and

means responsive to the intensity of radiation detected for indicating the concentration of gaseous anesthetic in said mixture.

5. In combination:

a system of inhalation anesthesia in which a mixture of oxygen and an anesthetic gas is supplied to a patient;

meansfor transmitting radiation of a wavelength shorter than 3000 A. through a portion of said mixture;

means for selectively detecting radiation of a wavelength shorter than 3000 A. after said radiation has passed through a portion of said mixture; and

means responsive to the intensity of the radiation detected for generating a signal that varies in accordance with the concentration of said anesthetic gas in said mixture.

6. In a system of inhalation anesthesia in which a mixture of oxygen and an anesthetic gas is supplied to a patient, the combination of:

a breathing mask adapted to supply such mixture to a patient; means including a supply line connected to said mask 5 for supplying such mixture to the patient; an auxiliary line connected to said supply line and leading to a Waste; a gas test cell located in said auxiliary line; means for transmitting radiation through a portion of 10 the gas in said gas test cell; means for detecting such radiation after passage through said portion of said mixture; and

References Cited in the file of this patent UNITED STATES PATENTS Heidbrink June 21, 1938 Van den Akker May 30, 1944 Cherrier Feb. 28, 1956 Bergson Mar. 17, 1959 

4. IN A SYSTEM OF INHALATION ANESTHESIA IN WHICH A MIXTURE OF OXYGEN AND A GASEOUS ANESTHETIC ARE FED THROUGH A SUPPLY LINE A PATIENT, SAID ANESTHETIC HAVING AN ABSORPTION SPECTRUM WITH AN EDGE IN A REGION IN WHICH OTHER GAS COMPONENTS FLOWING IN SAID LINE ARE SUBSTANTIALLY FREE OF ABSORPTION: MEANS FOR MEASURING THE ABSORPTION OF RADIATION BY SAID ANESTHETIC GAS IN A NARROW BAND ADJACENT SAID ABSORPTION EDGE, SAID MEANS COMPRISING A RADIANT-ENERGY SOURCE AND A DETECTOR MOUNTED ON OPPOSITE SIDES OF 